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Ultrafast Photoprocesses
in Nucleic Acids: New Insights into Ancient Sunscreens
Kohler, Bern1,
Pecourt, Jean-Marc1 and Peon, Jorge1
Ohio State University1
Abstract-
Femtosecond pump-probe spectroscopy was used to study the dynamics of
the lowest singlet excited states in DNA and RNA nucleosides in room
temperature aqueous solution. Excited state absorption at visible probe
wavelengths decays in hundreds of femtoseconds due to internal conversion
to the electronic ground state. Internal conversion produces vibrationally
highly excited ground state molecules that cool by intra- and intermolecular
vibrational energy transfer to the solvent on the timescale of a few
picoseconds. The rate of excited state decay is unique for each DNA
base, and the purines have higher rates of decay consistent with their
smaller propensity for photochemical damage. This work is the first
direct observation of ultrafast nonradiative decay in DNA components.
This process is responsible for DNA's high intrinsic photostability.
In essence, the DNA bases function as 'primordial sunscreen'
, and this fact may have played a critical role in early molecular evolution
before the advent of earth's ozone layer.
Keywords: DNA
photodamage, Excited state dynamics, Ultrafast spectroscopy
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